Throughout
most of the20th century,
researchers developing the synthetic theory of evolution primarily focused on
microevolution,
which is slight genetic change over a few generations in a population. Until
the 1970's, it was generally thought that these changes from generation to generation indicated that past
species evolved gradually into other species over millions of years.This model of
long term gradual change is usually referred to asgradualism or phyletic gradualism . It is essentially the 19th century
Darwinian idea that species evolve slowly at a more or less steady rate. A natural
consequence of this sort of macroevolution would be the slow progressive change of
one species into the next in a line, as shown by the graph on the right.

Punctuated equilibrium

Beginning
in the early 1970's, this model
was challenged by Stephen J.
Gould, Niles Eldredge, and a few other leading paleontologists. They
asserted that there is
sufficient fossil evidence to
show that some species remained essentially the same for
millions of years and then underwent short periods of very rapid,
major change.
Gould suggested that a more
accurate model in such species lines would be punctuated equilibrium (illustrated
by the graph on the left).

Long periods of stabilityandshort episodes of change

The punctuated, or rapid change periods, were presumably
the result of major environmental changes in such things
as predation pressure, food supply and
climate. During these times, natural selection can favor varieties
that were previously at a comparative disadvantage. The result can be
an accelerated rate of change in gene pool frequencies in the direction of
the varieties that become the most favored by the new environmental
conditions. It would be expected that long severe
droughts, major volcanic eruptions, and the beginning and ending of
ice ages would be likely triggers for rapid evolution.
In such stressful situations, populations would be expected to initially
diminish and become isolated. Genetic drift would then potentially
speed up the rate of evolution. If by chance nature favored successful
adaptations, the population would again increase in numbers as a radically changed
species. Conversely, if it favored maladaptive variations, the
population would decrease in numbers further and possibly even become
extinct.

Random mutations
provide variations that help a species survive. Mutations in
regulator genes
in particular can
quickly result in radically new variations in the organization of the body and its
important structures. As a consequence, changes in
these genes can result in a greater likelihood that at least
some individuals will have variations thatwillallow them to survive during times of extinction
level events. In this situation, subsequent generations would be
significantly changed from the generations before the period of severe natural
selection. In other words, regulator genes probably
play an important part in the rapid change phases of punctuated evolution.

Short-lived species with quick
generation replacement times usually evolve at a faster rate than do large,
long-lived species. This is because new genetic variations normally
appear each generation as a consequence of mutation in sex cells.
Those variations may be selected for or against depending on the environment
at the time. As a consequence, quicker reproductive cycles generally
result in speeded up species divergence. It is not surprising
that there are far more species of insects and microscopic organisms than
species of large trees and big animals such as elephants, horses, and
humans.

Tropical species also generally
evolve at a faster rate than do those from colder temperate climates.
Subsequently, tropical forests are more diverse ecosystems than forests in
colder regions. This is probably because warm environments promote
shorter generation times and higher mutation rates.

A relatively new but extremely
important factor in affecting rates of evolution has been people.
There are now nearly 7 billion of us, and our numbers are growing rapidly. We have already severely
changed most environments on our planet to suit our needs. In
addition, we are the super predator around the globe, bringing many species
to the brink of extinction and beyond. As a consequence, humans have
dramatically altered natural selection. The surviving animal and plant
species have responded to this pressure in a variety of ways. For
instance, fish species that are heavily exploited by people now usually have
smaller bodies as adults and begin to reproduce at an earlier age. It
is also likely that because humans increasingly live in urban environments
and rely on ever more technology, the evolution of our species has
accelerated and changed in ways that are yet to be discovered.

It is apparent that the evolutionary history of life on this planet is extremely
complicated. Different species have evolved at different rates and those rates have
changed through time in response to complex patterns of interaction with other species and
other environmental factors.In addition, it is clear that most species lines have
already become extinct as a result of their inability to adapt to changed conditions.

Origin of Species

Where do new species come
from? That is a key question that the biological sciences have been
asking for more than 200 years. Charles Darwin gave us part of the
answer in his explanation of natural selection. The
remainder came as a result of Gregor Mendel's experiments with basic genetic
inheritance and the 20th century discoveries of the other
natural processes that can cause evolution. We now know that evolution
can occur in two different patterns: adaptive radiation
and successive speciation.

Adaptive radiation
is the
progressive diversification of a
species into two or more species as groups adapt to different environments.
Natural selection is usually the principle mechanism driving adaptive
radiation. The initial step is the separation of a
species into distinct breeding populations. This usually happens as a
result of geographic or social isolation. Over time, the gene pools of
the isolated populations diverge from each other by gradually acquiring
different mutations and sometimes as a result of random genetic drift. When the populations are in dissimilar
environments, environmental stresses are often not the same. As a
result, nature selects for different traits existing within the
gene pools of the now cut off populations. Over time, the populations genetically diverge enough
so that they can no longer reproduce with each other. At this point,
they have become separate species and usually continue to diverge in
subsequent generations. In intermediate stages, the
two newly or about to be separated species may be able to interbreed and
produce children, but most of them are likely to be sterile. This is the
case with the offspring of female horses and
male donkeys--i.e., mules. Eventually,
however, species genetically diverge so much that they are unable to produce
any offspring. This is the case with sheep and cattle.
The process of adaptive radiation results in a branching evolutionary
pattern known as cladogenesis.

Adaptive radiationresulting in cladogenesis

The evolution of species
by
successive
speciation occurs within a single evolutionary
line without the branching of adaptive radiation. This
takes place when the
members of a species consist of a single breeding population for many
generations. Descendant generations experience continuous spontaneous
mutations and new directions of natural selection as the environment
changes. This results in progressive changes in the gene pool
frequencies of the population. At any one time, all members of the
population are the same species. However, as generations subsequently
replace each other, the gene pool is transformed--i.e., it evolves. Eventually,
the changes are great enough that if descendants could go back in time to mate
with their distant ancestors, the genetic differences would prevent them from
producing fertile offspring. In other words, they would be different
species. The process of successive speciation results
in a non-branching evolutionary pattern known as anagenesis.

Successive speciationresulting in anagenesis

In
the real world, the patterns of evolution can be very complex and
changing. Both adaptive radiation and successive speciation can go on simultaneously.

Origin
of Life

It may seem strange that the question of the ultimate origin of life on earth was not
discussed at the beginning of this tutorial. It was an intended
omission. The focus has been on the
processes by which living things change through time, not on how life first
came about. These are separate issues. A consideration of
ultimate origins bridges into the realm of religion for many people.
Regardless of whether you believe that life began spontaneously as a result
of natural processes or was due to divine intervention, it is sobering to
realize that science is close to being able to create life out of non-living
substances. In fact, most of the initial steps have already been
taken. The video linked below shows just how close we are to creating living organisms.

Revealing the Origins of Life--excerpt
from the PBS series Nova Science Now (February
16, 2011)This link takes you to an external
website. To return here, you must click the "back" button
on
your browser program. (length =
10 mins,
50 secs)